This invention pertains generally to the field of semiconductor diode lasers and particularly to vertical-cavity surface-emitting lasers.
Vertical-cavity surface-emitting lasers (VCSELs) have several significant advantages including low-threshold, high-fiber coupling efficiency, and a compact size that is well suited to integration. Single-mode VCSELs with output powers in the 5-20 mW range would be especially useful for applications such as laser printing (emission wavelength xcex=0.780 xcexcm) and telecommunications (xcex=1.3-1.55 xcexcm). Wet-oxidized VCSELs having single-mode power in the 3-5 mW range at xcex=0.85 xcexcm have been developed. K. D. Choquette, et al., 1997 Summer Topical Meeting on Vertical-Cavity Lasers, Montreal, Quebec, Canada, August, 1997; C. Jung, et al., Electron. Lett. Vol. 33, 1997, pp. 1790 et seq. However, because of their weak (positive-) lateral index-guiding nature, such VCSELs are very susceptible to gain spatial hole burning and thermal waveguiding, making the VCSEL aperture for single-mode operation very limited in size. To date, the highest fundamental-mode cw output power is 4.8 mW from a 3.5-xcexcm-diameter oxidized VCSEL, achieved by placing the oxide aperture at the optical field standing-wave null position. See, C. Jung, supra.
To obtain higher single-mode powers, the use of a negative-index guide (antiguide) is beneficial. Antiguides have demonstrated high-power, single-mode operation, from edge emitting lasers. D. Botez, et al., Appl. Phys. Lett., Vol. 53, 1998, pp. 464 et seq. More recently, antiguides have been implemented in VCSELs. See, Y. A. Wu, IEEE J. Sel. Top. Quantum Electron., Vol. 1, 1995, pp. 629 et seq.; T. H. Oh, IEEE Photonics Technol. Lett., Vol. 10, 1998, pp. 12 et seq.; K. D. Choquette, et al., Electron. Lett., Vol. 34, 1998, pp. 991 et seq. The advantage of an antiguide structure is that it provides strong lateral radiation losses, which are highly mode dependent, thus filtering out higher order spatial modes (even for large diameter devices, dxe2x80x2 greater than 6 xcexcm). In addition, a large index-step (xcex94n greater than 0.05) provides for mode stability against carrier and thermally induced index variations. Antiguided VCSELs have been fabricated either by surrounding a low index core region by regrowth of a high-index material (Y. A. Wu, supra) or by creating a low-index core region by shifting the cavity resonance (toward longer wavelengths) outside of the core (T. H. Oh, supra; K. D. Choquette, Electron. Lett. 34, supra). The latter structure relies on the cavity induced index step as proposed in G. R. Hadley, Opt. Lett., Vol. 20, July 1995, pp. 1483 et seq. These devices display promising results, including single-mode operation up to 5-15xc3x97Ith for diameters as large as 16 xcexcm. On the other hand, the power has been limited to less than two mW because of the relatively large radiation loss incurred for the fundamental mode, which is inherent to the antiguide structure. See, R. W. Engelmann, et al., IEEE Proc., Part I: Solid-State Electron Devices, Vol. 127, 1980, pp. 330 et seq.
In addition to the advantages in performance over conventional edge-emitting semiconductor lasers for data communications applications (including low divergence, circular beam, and dynamic single frequency operation), VCSELs also have a simplified fabrication process as compared to edge emitting distributed feedback-type lasers and allow wafer level characterization, which provide significant advantages in the manufacturing of such devices. A shortcoming of current VCSELs is the low power output (2-3 mW) for single-mode operation. Higher power single-mode operation would be desirable for applications ranging from telecommunications sources in the 1.3-1.55 xcexcm wavelength range to sources for optical recording and digital video disks (DVDs) in the visible wavelength range (630-650 nm).
Vertical cavity surface emitting semiconductor lasers formed in accordance with the present invention are capable of providing single mode output at higher power and with larger apertures than conventional VCSEL devices. Relatively high power is obtained by efficient lateral waveguiding of the fundamental mode while filtering out higher order spatial modes to achieve substantially single mode operation with less susceptibility to gain spatial hole burning and thermal waveguiding than conventional VCSELs.
The surface emitting semiconductor laser of the invention includes a semiconductor substrate, such as GaAs, and a multilayer structure on the substrate including a layer with an active region at which light emission occurs, upper and lower layers surrounding the active region layer, upper and lower faces, electrodes by which voltage can be applied across the multilayer structure and substrate, a central core region, and an antiresonant reflecting waveguide ring surrounding the central core formed to have an effective higher index than the central core and sized to provide antiresonant lateral waveguiding confinement of a fundamental mode lateral wavelength. An upper reflector is formed above the ring and active region layer and a lower reflector is formed below the active region layer to provide vertical confinement. The reflecting ring and the upper and lower reflectors are positioned to act upon the light generated in the active region to produce lasing action and emission of light from at least one of the upper and lower faces of the semiconductor laser. Typically, one of the reflectors, which are preferably formed as multiple layer distributed Bragg reflector mirrors, is partially transmissive to allow passage of light therethrough to, e.g., the upper face of the laser. The antiresonant reflecting waveguide ring can be formed to have a full ARROW structure including a high effective index region of odd multiples of a quarter lateral wavelength (including a single quarter lateral wavelength) and a low effective index region of odd multiples of a quarter lateral wavelength, or to have a simplified ARROW structure having a single high effective index region of odd multiples of a quarter lateral wavelength. The high effective index regions in the reflecting ring are preferably formed to locally increase the cavity resonance wavelength to provide an effective equivalent increase in the index in the ring. The structure further preferably includes a means for confining the current from the electrodes to the central core region, such as by use of proton implantation in the region surrounding the central core for the simplified ARROW structure or by utilizing a current blocking layer in the epitaxial structure that laterally surrounds the central core region and the reflecting ring. In addition, lossy layers can be placed in the reflector regions to suppress the guided modes of the ring waveguides.
The semiconductor laser of the invention may be formed with various material systems commonly used in semiconductor lasers. The active region is preferably formed to be aluminum free, e.g., as a multiple quantum well active region formed of layers of GaAs and InGaAs. The higher effective index region of the reflecting ring may be formed for such a material system of, e.g., layer of GaInP and a layer of GaAs.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.